Frequency response of an electroacoustic transducer



April 19, 1966 E. MARTIN 3,246,721

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FREQUENCY RESPONSE 0F AN ELECTRo-ACOUSTIC TRANSDUCER Filed April 24,1963 2 Sheets-Sheet 2 .as a coupling to the other resonator space.

United States Patent S 6 Claims. (Cl. 18131) The invention disclosedherein is concerned with an arrangement for improving the frequencyresponse of an electro-acoustic transducer. The transducer according tothe invention comprises a housing which is, with the aid of a membraneand a partition wall, which wall is provided with at least onebreak-through, subdivided to'form a space respectively in front and inback of the membrane and also a resonator space.

"It has been customary for many years to provide, in electro-acoustictransducers, resonator-s for the purpose of improving the frequencyresponse. There are absorption resonators known which absorb aresonance'peak of the frequency response and also resonators which areoperative to raise the frequency response in the lower part of thetransmission range. Such resonators are coupled either to the space infront or the space in back of the membrane.

The present invention introduces, for an electro-acoustic transducer ofthe initially indicated kind, a particularly advantageous arrangementfor an auxiliary resonator space. This arrangement, as compared withprior resonator arrangements, permits with simple measures a coupling tothe frontal space of the membrane as well It is also possible to effectsimultaneous coupling to the two indicated spaces, thereby producing aparticularly favorable action with respect to the frequency response.

The characteristic feature of the invention resides in the provision ofa further cylindrical partition wall, be-

tween the first partition wall and the membrane, so as to form anauxiliary annular resonator space which is sealed with respect to therearward space of the membrane.

The foregoing and further objects and features of the invention willappear from the description of embodiments which is rendered below withreference to the accompanying drawings.

FIG. 1 shows the principles applied in an arrangement according to theinvention, with the auxiliary resonator space provided with an openingto the frontal space of the membrane;

FIG. 2 represents the electrical analog or equivalence circuit for thearrangement according to FIG. 1;

FIG. 3 indicates a constructive realization in connection with anelectromagnetic transducer;

FIGS. 4 and 6 illustrate examples of the arrangement according to theinvention, wherein openings are provided leading from the auxiliaryresonator space to the original resonator space which is coupled to therearward space of the membrane, and with openings to the latterresonator space and the frontal space of the membrane; and

FIGS. 5 and 7 represent electrical analog or equivalence circuitsrespectively for the embodiments illustrated in FIGS. 4 and 6.

Accordingly, FIGS. 1, 3, 4 and 6 show the spatial arrangement of partsand FIGS. 2, 5 and 7 show electrical equivalence circuits drawn inaccordance with the force-voltage analogy.

FIG. 1 shows an electro-acoustic transducer having a housing G which isdisposed within the chamber HA of a telephone hand set. The transducerhousing G is by the membrane M and the partition wall T1, which tureprinciple. shown (in FIG. 3) so as to give a practical example for theapplicability of the arrangement illustrated in is provided with abreak-through L1, subdivided to form the frontal space C4 and therearward space C2 of the membrane, and also the resonator space C1. Thebreakthrough L1 in the partition wall T1 is closed by an acousticresistor R1.

The sound passage openings of the transducer housing are contiguous tothe sound passage channels L3, R3 which extend to the outside. It is inthe present case assumed that the electro-acoustic transducer is alsoused as a receiver. There is therefore indicated an aural volume C3.According to the invention, there is provided, by a further cylindricalpartition wall T2 extending between the first partition wall T1 and themembrane M, an auxiliary resonance space C5 which is sealed with respectto the rearward space of the membrane. This auxiliary resonance space C5is in this embodiment connected with the frontal space of the membraneover the annular acoustically operative channel L6, R6.

FIG. 2 represents the electrical analog or equivalence circuitcorresponding to the acoustic conditions of FIG. 1. The capacitancescorrespond to the reciprocal stilfnesses of the respective volumes andthe inductances and resistances correspond respectively to the massesand the acoustic resistances of the respective openings. The elementsindicated by the sign refer to the membrane. The branch R'6, L6, C'5,corresponding to the solution according to the invention, whichdistinguishes over the known networks, by the parts indicated byprominent lines.

If the volume of the auxiliary resonator space C5 is relatively great,the capacitance of the capacitor C5 will likewise be relatively high. Itacts in a manner similar to a shunt, so that the series circuit of theinductance L6 and the resistance R'6 lies parallel to the capacitanceC4, making it possible, with appropriate dimensioning of the inductivityand the resistance, that is, with appropriate dimensioning of theacoustically active opening at the rim of the membrane, to effect araising in the lower part of the frequency band.

If the volume of the auxiliary resonator space C5 is relatively small,there may be obtained, by appropriate dimensioning of the opening L6, R6(FIG. 1), at the rim of the membrane, a resonator which is tuned to afrequency in the central or upper part of the transmission range. Thisresonator acts in such case as an absorption resonator and can be usedfor the attenuation of a resonance peak in the frequency response of thetransducer.

The arrangement of the auxliary resonator space is particularlyimportant in connection with electromagnetic transducers operating inaccordance with the ring arma- Such a transducer is therefore likewiseFIG. 1. The annular support A for the ring shaped armature RA isdimensioned and arranged so as to leave an intermediate space betweenthe permanent magnet D and the support A, such space being utilized asauxiliary resonator space C5. The acoustically effective connection withthe frontal space of the membrane is established by one or more cutoutsL6, R6, formed at the outer rim of the membrane.

The structure according to FIG. 4, in which the auxiliary resonatorspace C5 is not connected with the frontal space C4 of the membrane but,over an acoustically effective opening L7, R7, with the resonator spaceC1 in back of the membrane, produces an effect which will now bedescribed with reference to FIG. 5.

With appropriate translation and size of the auxiliary resonator spaceC5, C5 in FIG. 5 can be neglected. The oscillation structure C2, L'l,C'l, L'7 can then be tuned to two resonance peaks within thetransmission range. Upon tuning the weakly attenuated HelmholtzresonatorL1, C1, to the membrane resonance (about 2000 cycles) and the auxiliaryresonator to the lower resonance rise (about 1400 cycles) of theHelmholtzresonator, there will be obtained a linearized frequency coursewithout requiring attenuation of the Helmholtzresonator with the aid ofsilk. The relatively slight attenuation due to the friction of air atthe resonance neck is in such a case sufficient. This condition isindicated in FIG. by dash-line representation of the acoustic resistanceR]. (R1 in FIGS. 1 and 4).

In the embodiment shown in principle in FIG. 6, there is provided, inaddition to the acoustically effective connection L6, R6, between theauxiliary resonator space C5 and the space in front of the membrane, thefurther acoustically effective connection L7, R7, extending between theauxiliary resonator space C5 and the original resonator space C1. Theoperation obtained thereby may be explained with reference to theelectrical analog circuit representation shown in FIG. 7.

In FIG. 7, the openings referred to, represented by the inductances L6and L'7, as well as the resistances R6 and R7, and the volume of theauxiliary resonator space, represented by the capacitance (3'5, areshownin prominent lines. The analog circuit represents an approximationfor cases in which (3'1 is considerably greater than C4 and wherein thetwo openings in the auxiliary resonator have about the samecross-sectional area.

The case in which the volume of the auxiliary resonator space C5 isrelatively small, is of interest. In such case, the leakage over thecapacitance C'S can be neglected, so that the inductances U6 and L7 arein series relationship. They form, jointly with the capacitances U1 andC4, a parallel oscillation circuit which can easily be tuned to lowfrequencies even in the case of small mass of the resonator openings.due to serial relationship of the two inductances L6 and L7. Such tuningpermits raising of the frequency response in the low part of thetransmission range.

With appropriate size of the auxiliary resonator space C5, the measureindicated in FIG. 6 will not only permit tuning at low frequencies andtherewith raising the frequency course in the lower part of thetransmission range, but will, with appropriate dimensioning of theopening L6, R6, effect an absorption at one resonance peak in thetransmission range. In such case, the series resonance of the elementsL'6, R6, C'S (FIG. 7) becomes effective.

Changes may be made within the scope and spirit of the appended claimswhich define what is believed to be new and desired to have protected byLetters Patent.

I claim:

1. In an electro-acoustic transducer having a housing which is by amembrane and a partition wall, which wall is provided with at least onebreak-through, subdivided into'a space formed respectively in front ofthe membrane and in back thereof, an arrangement for improving thefrequency response of the transducer, comprising a further, cylindrical,partition wall disposed between the first This is substantially 4 namedpartition wall and the membrane, said further partition wall forming anauxiliary annular resonator space which is sealed thereby with respectto the laterally adjacent space in back of the membrane.

2. An arrangement according to claim 1, wherein said auxiliary resonancespace communicates with the space in front of the membrane by way of atleast one acoustically effective opening formed at the rim of themembrane.

3. An arrangement according to claim 2, wherein the outer diameter ofthe membrane is somewhat smaller than the inner diameter of the housingat the level of the membrane, thus forming an annular gap between thenamed parts.

4. An arrangement according to claim 2, wherein the membrane isperipherally thereof at least in part perforated.

5. An arrangement according to claim 2, wherein the auxiliary resonatorspace communicates with a resonator space coupled to the space in backof the membrane, by Way of at least one acoustically effective opening.

6. In an electro-acoustic transducer, the combination of a housingincluding a front wall portion, a rear wall portion, and peripheral wallmeans connecting the front and rear wall portions, a membrane and apartition wall extending between the peripheral wall means andsubdividing the housing interior to form a first space between saidmembrane and said front wall portion, and a second space between saidmembrane and said partition wall, the latter having an opening therein,and a further tubular partition wall extending between thefirst-mentioned partition wall and the membrane and cooperable therewithand with said peripheral wall means to define an annular resonator spacewith said tubular partition wall forming a seal between said resonatorspace and the laterally adjacent space between said membrane and thefirst-mentioned partition wall, said annular resona tor space having anopening in a wall defining the same, providing communication betweensuch annular resonator space and an adjacent space Within said housing.

References Cited by the Examiner UNITED STATES PATENTS 1,854,716 4/1932Pape 181--31 2,327,137 8/1943 Shapiro 179-107 2,342,334 2/1944 Faltico181-31 2,490,466 12/ 1949 Olson et al 18131 2,524,393 10/1950 Lybarger179-107 2,585,052 2/ 1952 Skelton.

2,778,882 1/1957 Pontzen et al 179--115.5

FOREIGN PATENTS 1,078,174 3/1960 Germany.

LEO SMILOW, Primary Examiner.

LEYLAND M. MARTIN, Examiner.

1. IN AN ELECTRO-ACOUSTIC TRANSDUCER HAVING A HOUSING WHICH IS BY AMEMBRANE AND A PARTITION WALL, WHICH WALL IS PROVIDED WITH AT LEAST ONEBREAK-THROUGH, SUBDIVIDED INTO A SPACE FORMED RESPECTIVELY IN FRONT OFTHE MEMBRANE AND IN BACK THEREOF, AN ARRANGEMENT FOR IMPROVING THEFREQUENCY RESPONSE OF THE TRANSDUCER, COMPRISING A FURTHER, CYLINDRICAL,PARTITION WALL DISPOSED BETWEEN THE FIRST NAMED PARTITION WALL AND THEMEMBRANE, SAID FURTHER PARTITION WALL FORMING AN AUXILIARY ANNULARRESONATOR SPACE WHICH IS SEALED THEREBY WITH RESPECT TO THE LATERALLYADJACENT SPACE IN BACK OF THE MEMBRANE.